282 ZOO-GEOGRAPHICAL DISTRIBUTION [CH. 



of distribution. They are always small in number of species, 

 with related forms widely separated, and usually only occupying 

 small isolated areas. Thus the total fauna of a region resembles 

 a picture produced by the "three-colour process," in which the 

 three separate elements, palaeogenic, entogenic, and ectogenic, 

 merge into one complete whole. 



A number of genera have become so firmly established in more 

 than one region, that they may be considered to have formed 

 definite zoocentres in two or more of them. These are termed 

 polyentogenic groups (di-,tri-,tetra-, penta- or even hex-entogenic). 

 Special cases of these are known as Holarctic (2), New World (2), 

 Old World (4), Austro-Malayan (2), Bipolar (4), Circumtropical (4), 

 or Cosmopolitan (6) groups. In the table of entogenic genera, 

 these are included under the regions in which they are entogenic, 

 with a number in brackets following each genus, to indicate in 

 how many regions it may be considered to be so ; e.g. Orthetrum (3), 

 Pantala (6). 



We shall now proceed to discuss separately the palaeogenic, 

 entogenic and ectogenic Odonate faunas of the World. 



The Palaeogenic Fauna. 



From the definition of palaeogenic groups, we see that they 

 should not be particularly associated with any one region. Even 

 if to-day a palaeogenic group is only found in one region, we 

 indicate, by including it in this fauna, that we believe it to be, 

 not a true portion of the entogenic fauna of that region, but the 

 remains of a fauna that was once far more widely spread. There 

 are five isolated groups which may be regarded as palaeogenic. 

 Four of these are subfamilies, and one is a tribe : 



1. Subfamily Petalurinae. This very ancient group has 

 to-day a very typical palaeogenic distribution. In Neotropica, 

 the genus Phenes (1 species) is confined to S. Chili. In Nearctica, 

 the genus Tachopteryx has one species in Nebraska, and one species 

 in New York and Pennsylvania. In Palaearctica, the same genus 

 has one species in Japan. In Australia, the principal genus 

 Petalura is represented by three species; while in New Zealand 

 the genus Uropetala is represented by one species. All the species 

 are of large size, occupy only small areas, and bear all the marks 



